Issue 33

J. Toribio et alii, Frattura ed Integrità Strutturale, 33 (2015) 221-228; DOI: 10.3221/IGF-ESIS.33.28

E XPERIMENTAL RESULTS

Microstructural analysis igure 3 shows the microstructure of hot rolled bar and prestressing steel wire, longitudinal section, where the horizontal size of the micrograph corresponds to the radial direction and the vertical size is associated with the axial direction in the longitudinal cut. The drawing process produces important microstructural changes in the steel at the two basic microstructural levels of pearlitic colonies and lamellae. The colonies become progressively enlarged and oriented in axial direction with cold drawing. With regard to the lamellae, they are also axially oriented after drawing and, at the same time, the pearlite interlamellar spacing decreases with the level of cumulative plastic strain. Therefore, the microstructure becomes progressively packed and oriented with cold drawing. F

Figure 3 : Microstructure, longitudinal section, B0 (left) and B7 (right).

Fracture surface Fig. 4 shows the fracture surface for the different steels, where the fatigue surface, starting from a small mechanical cut (on the left side of the photographs), appears with a semi-elliptical final front from which the fracture initiates. Macroscopically, the fracture surfaces of the slightly drawn steels propagate perpendicularly to the applied stress, following the fatigue crack. On the contrary, in heavily drawn steels, the fracture is very anisotropic, with abundant deflections and longitudinal cracking. Symmetrical longitudinal cuts were made in the fracture-tested specimens, they were photographed (Fig. 5), and the fracture angle was measured. In these photographs the fatigue growth is shown on the left, with a flatter profile, in addition to the fracture appearing after such a fatigue growth. As the plastic deformation increases with the number of drawing steps, so does the roughness of the fracture surface, as well as the fracture angle θ (in relation to the cross section of the wire), which implies the occurrence of a fracture in mixed mode. Furthermore, in heavily-drawn steels, the existence of a step oriented with an angle of 90º (in relation to the cross section of the wire) can be observed at the beginning of the fracture event.

Figure 4 : Fracture surfaces, B0 to B7.

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